1. The Field of the Invention
The present invention concerns a method of predicting the maximum running distance in degraded mode, without substantial deterioration in the running conditions, of a mounted assembly comprising a wheel rim, a safety support mounted on the rim, and a tire cover mounted on the rim, with the support supporting the tread of the cover during such running. (Running xe2x80x9cin degraded modexe2x80x9d means running at reduced or zero inflation pressure.) The invention also concerns an installation for implementing this method.
2. The Related Art
It is known that the safety supports for vehicle tires are intended to be mounted on a rim inside a tire, with a view to being able to support the tread of the tire in the event of loss of inflation pressure. These supports comprise, notably, a base which is intended to be mounted on the rim, and a crown which is intended to come into contact with the tread in the aforementioned case (loss of inflation pressure) and which leaves clearance with respect to it at nominal pressure.
The international patent document WO-A-00/76791 (United States Published Application US 2002/0124924 A1, the disclosure of which is hereby incorporated herein for all purposes) presents such a support, in which the base and crown are substantially cylindrical, and which also comprises an annular body connecting the base and the crown. This annular body comprises a support element which is continuous circumferentially with a circumferential mid-plane. The support element comprises a plurality of partitions extending axially on each side of the circumferential mid-plane and distributed over the circumference of the support.
The tests or methods used at the present time for predicting the maximum running distance in degraded mode, without substantial deterioration of the running conditions, of a mounted assembly comprising a wheel rim provided with such a safety support and a tire cover mounted on the rim consist generally of:
running a motor vehicle equipped with such mounted assemblies in degraded mode on a circuit of the road or motorway type, at a predetermined constant speed (for example around 100 km/h) and at a given external temperature, and then
interrupting the running when the driver of the vehicle detects such a substantial deterioration of the running conditions that continued running in degraded mode is very difficult, a deterioration which is due to significant damage to the mounted assemblies and which results, for example, in a substantial increase in the vibration originating at the steering wheel, or following an examination of each mounted assembly following running flat over predetermined distances.
Generally, the criterion for stopping the running test in degraded mode, which is chosen by the operator in charge of the test, corresponds to the appearance of one or more specific items of damage concerning both the safety support and the tire cover.
The damage concerning the support can, for example, consist of cracks or breaks at the partitions of the support because of significant internal heating and buckling stresses to which the running support is subjected in degraded mode.
The damage concerning the tire cover can, for example, consist of cuts at the sidewalls of the tire cover, notably because of the camber stresses to which the tire cover is subjected on a more or less winding circuit, or by a burst pure and simple thereof, making it impossible to continue any running in degraded mode.
However, experience shows that these stoppage criterion or criteria are parameters that can have a determining effect on the result with regard to the maximum running distance in degraded mode without substantial deterioration in the running conditions, which is obtained at the end of the test on a circuit.
The same applies to the parameters characterizing the running which are peculiar to the vehicle, such as the speed chosen for the running or the load to which each mounted assembly is subjected during running.
Naturally, the parameters relating to the ambient air (temperature) and to the surface of the circuit used (roughness, dry or wet ground) may also have an influence on the maximum running distance obtained in degraded mode.
A major drawback of these prediction tests on a circuit lies in the difficulty of keeping the aforementioned parameters identical from one test to another because of their variability, and in the more or less constraining character of these parameters for the support and tire cover during running in degraded mode. This may result, in particular, in difficulties in comparing the respective running endurances of various mounted assemblies in degraded mode.
An object of the present invention is to propose a method of predicting the maximum running distance in degraded mode, without substantial deterioration of the running conditions (that is to say without loss of control of the vehicle), of a mounted assembly comprising a wheel rim, a safety support mounted on the rim, and a tire cover mounted on the rim around the support, with the support supporting the tread of the cover during the running, which makes it possible reliably and reproducibly to predict the maximum running distances in degraded mode of various mounted assemblies and to compare them with each other under identical experimental conditions.
To this end and according to a first embodiment of the invention, the prediction method comprises running the mounted assembly at a reduced or zero inflation pressure, from a time t0, at a given temperature, at a given mode and at a constant speed V, on at least one running surface so that the center of the wheel rim is a substantially invariant point during the running (i.e. on a rolling road test drum, typically), monitoring the variation in a variable R representing the radial loading of the support as a function of the reduced or zero pressure running time t, and, during such running, implementing the following sequence of steps (i) to (iii):
(i) determining a value R1 attained by the variable R at the end of a predetermined stabilization time t1 which is such that the direction of variation of the variable R represents a radial loading of the support increasing overall beyond the stabilization time t1, then
(ii) determining a critical running time t2 (t2 greater than t1) at the end of which the variable R reaches a critical value R2 such that R2=R1+xcex94R, where xcex94R is a value representing a critical increase in the loading of the support with respect to the value R1 at the end of the stabilization time t1, and then
(iii) making the running time t2 correspond to a distance d2, with d2=V(t2xe2x88x92t0), representing a prediction of the maximum running distance without substantial deterioration in the running conditions.
It should be noted that the value xcex94R which is adopted at step (ii) constitutes a criterion for stopping the running, beyond which the support is subjected to stresses and heating liable to make it unsuitable for use.
It should also be noted that it would be possible to choose at least one new critical value xcex94Rxe2x80x2 greater than or less than xcex94R according to the absence or presence of substantial damage in the support at the end of the time t2, and once again to implement the sequence of steps (i) to (iii) by replacing xcex94R with xcex94Rxe2x80x2, so as to obtain, at the end of n iterations, a still further improved prediction of the maximum running distance of the support without substantial deterioration in the running conditions.
According to a second embodiment of the invention, the method of predicting the maximum running distance of the mounted assembly at a reduced or zero inflation pressure, without substantial deterioration in the running conditions, comprises running the support mounted on the wheel rim, also at a given temperature, at a given load and with a constant speed V, directly in contact with the running surface so that the center of the rim is a substantially invariant point during the running, monitoring the variation in the variable R representing the radial loading of the support as a function of the reduced or zero pressure running time t, and, during this running, implementing the aforementioned steps (i), (ii) and (iii).
Preferably, the support is mounted on the rim by snapping on, in this second mode.
According to a preferred example of implementation of the invention which is common to these two embodiments, the predetermined value xcex94R is such that, at the time t2, the rate of increase |dR/dt| in the loading of the support is greater than a given critical threshold.
According to another characteristic of the invention common to these two embodiments (i.e. running of the mounted assembly or only of the support on the running surface), step (ii) above comprises monitoring the variation in the variable R from the time t1, and predicting that it attains the critical value R2 at the critical time t2 substantially when the instantaneous acceleration of the loading d2R/dt2 of the support passes through a zero value.
It should be noted that this critical time t2 is such that the graph of the variable R exhibits a reversal point substantially at the time t2, that is to say, a reversal of the direction of variation in the slope dR/dt for t greater than t2, representing a higher and higher rate of loading of the support which rapidly results in the aforementioned cracking or rupture of the support.
Concerning the first embodiment of the invention, the variable R representing the radial loading of the support advantageously corresponds to the mean radius of the support during loading (also referred to as the xe2x80x9cloaded radiusxe2x80x9d), such radius being measured between a first point defined in the center of the wheel rim and a second point defined in the center of the contact surface between the tread and the running surface.
Concerning the second embodiment of the invention, this variable R also corresponds to the mean radius during loading, except that this radius is here measured between a first point defined in the center of the wheel rim and a second point defined in the center of the contact surface between the radially external face of the support and the running surface.
It should be noted that the direction in variation of these radii during loading decreases overall as a function of the running time t, beyond the stabilization time t1.
It should also be noted that the variable R could also correspond to the flexion relating to the support because of the loading, or to the relative loading of the support (the ratio of the flexion to the height of the support), the direction of variation of this flexion or this relative loading increasing overall as a function of the time t, beyond the stabilization time t1.
According to another advantageous characteristic of the invention concerning solely the aforementioned first embodiment, the prediction method also comprises estimating that the maximum running distance without substantial deterioration in the running conditions is reached just before smoke is detected inside the mounted assembly.
According to one advantageous embodiment of the invention common to the aforementioned two embodiments, the running surface used has a substantially cylindrical geometry, and comprises, for example, a rolling road test drum, i.e., one whose running surface is a cylinder with a circular cross section. It should be noted that this running surface can be convex or concave, depending on whether the external or internal face of the test drum, respectively, is used.
According to another exemplary embodiment of the invention common to the aforementioned two embodiments, the running surface used has a substantially flat geometry, for example, of the conveyer belt type.
Concerning one or other of these exemplary embodiments of the invention, it should be noted that the running surface used can be smooth, or have a plurality of projecting and/or recessed irregularities which are more or less regularly spaced apart on its perimeter. These irregularities can, for example, consist of obstacles of the bar type, intended to reproduce the running stresses due to manhole covers or other reliefs normally encountered during actual running on a road, or hollows, intended, for example, to reproduce the stresses inherent in running over potholes.
According to one advantageous exemplary embodiment of the invention, the wheel rim has at each of its peripheral edges a rim seat intended to receive a bead on the tire cover, the wheel rim having between its two seats, on the one hand, a surface intended to receive the support and, on the other hand, a mounting groove connecting the surface to an axially internal flange on one of the seats.
Reference can be made to the French patent document FR-A-2 720 977 (U.S. Pat. No. 5,836,366, the disclosure of which is hereby incorporated by reference for all purposes) for a detailed description of the mounting of the tire cover on the rim.
As for the support according to the invention, it is advantageously of the type having:
a substantially cylindrical base intended to be mounted on the rim,
a substantially cylindrical crown intended to come into contact with the tread of the tire cover in the event of a drop in pressure, and leaving a clearance with respect to the tread at nominal pressure, and
an annular body connecting together the base and the crown, the body having a circumferentially continuous support element with a circumferential mid-plane, the support element comprising a plurality of partitions extending axially on each side of the circumferential mid-plane and distributed over the circumference of the support.
An installation according to the invention for implementing the aforementioned prediction method according to the first or second embodiments comprises essentially at least one running surface, and one or more running stations which are each intended for the running on the running surface of a mounted assembly comprising a tire cover mounted on a wheel rim around a safety support with a reduced or zero inflation pressure, or for the running on the running surface of a support mounted on a wheel rim, the center of the mounted assembly or of the support being a point which is substantially invariant during the running on the running surface or surfaces, wherein such installation also comprises:
detection means connected to the running station or stations and which are designed to detect at all times, during the running on the surface or surfaces, the information representing the effects caused by the running, including at least one item of information representing the radial loading of the support at all times, and
a unit for controlling the starting of the running according to predetermined running parameters, including a running speed V and a load to be applied to the support during running, in order to receive the information from the detection means and to store it, and to control the stopping of the running if at least one of the items of information reaches a predetermined critical value.
According to another characteristic of the invention, the detection means comprises a loading sensor, for example of the potentiometric type, which is designed to provide at all times a value of the support radius during loading which represents the mean radial loading of the support during running, such radius during loading being measured between a first point defining the center of the wheel rim and a second point defining the center of the contact surface between the cover, or the support according to circumstances, and the running surface.
Preferably, in the case of a running of the mounted assembly on the running surface, the detection means also comprise a smoke detector which is designed to detect the presence of smoke through internal heating inside the mounted assembly during running at reduced or zero pressure, by suction means which are provided inside the running station in order to suck the, air included inside the mounted assembly in the direction of the detector.